The Minsk II cease-fire agreement brokered by Russia, Germany and France last month has changed the political dynamics in Europe. Germany and to a lesser degree France have resisted calls to provide lethal weapons to Kiev, and much of the rest of Europe takes its cue from Berlin and Paris. They did not want the arms issue to interfere with the Minsk negotiations, and they continue to oppose sending weapons to Ukraine as long as the basic cease-fire holds. They do not want to give a pretext to Mr. Putin and his proxies to move further into Ukraine.
Chancellor Angela Merkel is now widely associated with the cease-fire, and to some degree Germany is seen as a key guarantor that the agreement will be maintained. Some German politicians have quietly indicated that they would need to reconsider their overall policy should a fundamental breach of the cease-fire take place. A major Russian separatist attack on Mariupol would represent such a breach and would trigger a major European policy review.
Given this reality, a coordinated trans-Atlantic initiative could both unite the West and deter Mr. Putin from pursuing what is likely to be one of his key goals — the creation of a land bridge from Russia to Crimea. A Western initiative — call it “the Mariupol test” — would require the United States to reach agreement with Germany and the rest of Europe now on how the West would react should Mr. Putin make a move against the city. It would require the Obama administration to press its European allies to unite and deter Mr. Putin’s next move.
Russia has already violated the Minsk II cease-fire agreement by seizing the strategic transportation hub of Debaltseve a few days after the accord was reached. Although Western intelligence reports that Moscow is moving in more heavy weapons, the rest of the cease-fire line still seems to be holding, for now.
Along with many policy makers and analysts in Washington, we believe that providing lethal weapons to Kiev would show solidarity with Ukraine, make further military moves by Russia more painful, and deter Mr. Putin from aggressive steps elsewhere. Yet maintaining alliance unity in the face of the Kremlin’s challenge is critical to designing the kind of multifaceted response needed to prevent Ukraine from collapsing.
Mr. Putin is unlikely to honor Minsk II if he believes he can circumvent it without much cost. He is proud, emotional, aggrieved and shrewd — historically a dangerous combination. His skills, well-honed at the K.G.B., include a perfected ability to obfuscate and divide. His long-term goal may be the creation of “Novorossiya,” or New Russia, which would constitute all of southern Ukraine past Odessa to Moldova, and would enable Russia to control the entire northern coast of the Black Sea. There are no large armies to stop him.
If he succeeds, Ukraine would be a less-viable state, heavily dependent on Russia for access to the sea. The West would have failed to preserve Ukrainian independence or halt Russian aggression.
Mr. Putin may seek to create Novorossiya one slender slice at a time, thereby reducing his chances of massive confrontation with the West. An intermediate Kremlin goal would be to connect Crimea by land to Russia. Mariupol stands in the way. Ukrainian volunteers of all ages are digging trenches there to block what they believe will be Mr. Putin’s next move. The West must decide how to prevent Mariupol from becoming another bloody road bump on Mr. Putin’s westward drive.
American and European leaders urgently need to construct a viable deterrent to Mr. Putin’s plans for Novorossiya. This would not include NATO boots on the ground, since few in government or the public at large in Europe and America support that option. But any plan to contain Russia must be tough to be effective.
Mr. Putin has said he wants the cease-fire to hold. The West needs to unite and force him to keep that promise. It needs to construct a Mariupol test, which will turn that town from a road bump into a red line with teeth.
Washington and its European allies need to decide now what they will do should the separatist fighters and their Russian enablers who took Donetsk and Luhansk appear in Mariupol in force.
Under such circumstances, Kiev must be given lethal weapons and training, not just by the United States but by the Europeans as well. Tougher economic sanctions should also be imposed. The West should publicly discuss suspending Russia from the Brussels-based Swift financial-messaging system, a step which could cripple the already reeling Russian economy. Strict new limits on visas for Russian travel to the West should also be imposed. Such measures could ignite a dramatic reaction from Russia, such as a natural gas cut-off to Europe. But absent the will to introduce Western ground forces, it will take stern measures such as these to deter Moscow.
Should the cease-fire hold and Ukraine regain full control of its territory, then the West may start to ease sanctions. Mr. Putin will need incentives for him to back down. That can be done without accepting what the Kremlin has gained through violence.
Drawing the line at Mariupol in the manner we propose could constrain Mr. Putin enough to preserve the rule of law on the Continent. President Obama and Chancellor Merkel need to construct a deterrent proposal soon to make the “Mariupol test” a success.

Hans Binnendijk is a senior fellow at the Johns Hopkins Center for Transatlantic Relations John Herbst is director of the Atlantic Council’s Eurasia Center and served as U.S. ambassador to Ukraine from 2003 to 2006.